Inhibition of the proliferation of tumor cells is one of the main purposes of the therapy, especially the chemotherapy, of cancerous growths. A particular aspect of such proliferation is the phenomenon whereby some cells of the tumor break off into the bloodstream and travel to other parts of the body, where they produce secondary tumors; this phenomenon is termed metastasis. As is well-known, while there are many tumor-inhibiting substances either in actual use, or which have potential use, in the chemotherapy of cancer, nevertheless the problem of cancer as a whole is far from a solution. Thus, any new contribution to the art of chemotherapy provides the medical profession with an additional tool with which to fight this disease. As will be seen from the following description, the present invention provides a substance of biological origin which is believed to be useful in this context.